Method and apparatus for improving critical dimension variation

ABSTRACT

A method is described. The method includes obtaining a relationship between a thickness of a contamination layer formed on a mask and an amount of compensation energy to remove the contamination layer, obtaining a first thickness of a first contamination layer formed on the mask from a thickness measuring device, and applying first compensation energy calculated from the relationship to a light directed to the mask.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometric size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling-down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling-down has also increased the complexity ofprocessing and manufacturing ICs.

For example, there is a growing need to perform higher-resolutionlithography processes. One lithography technique is extreme ultravioletlithography (EUVL). The EUVL employs scanners using light in the extremeultraviolet (EUV) region, having a wavelength of about 1-100 nm. Onetype of EUV light source is laser-produced plasma (LPP). LPP technologyproduces EUV light by focusing a high-power laser beam onto small fueltarget droplets to form highly ionized plasma that emits EUV radiationwith a peak of maximum emission at 13.5 nm. The EUV light is thencollected by a collector and reflected by optics towards a lithographyexposure object, e.g., a substrate.

Although existing methods and devices for lithography process have beenadequate for their intended purposes, they have not been entirelysatisfactory in all respects. Consequently, it would be desirable toprovide a solution for reducing contamination on EUV masks in order toimprove critical dimension variation.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic view of a lithography system, in accordance withsome embodiments.

FIG. 2 is a cross-sectional view of a mask, in accordance with someembodiments.

FIG. 3 is a chart showing a relationship between a thickness of acontamination layer and a decrease in critical dimension, in accordancewith some embodiments.

FIGS. 4A-4C are charts showing how compensate energy is applied toreduce variation in critical dimension for a group of substrates, inaccordance with some embodiments.

FIGS. 5A and 5B are schematic views of the lithography system, inaccordance with alternative embodiments.

FIG. 6 is a schematic top view of a mask storage, in accordance withsome embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “over,” “on,” “top,” “upper” and the like, may be used hereinfor ease of description to describe one element or feature'srelationship to another element(s) or feature(s) as illustrated in thefigures. The spatially relative terms are intended to encompassdifferent orientations of the device in use or operation in addition tothe orientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

FIG. 1 is a schematic view diagram of a lithography system 10, inaccordance with some embodiments. The lithography system 10 may begenerically referred to as a scanner that is configured to performlithography exposure processes with respective radiation source andexposure mode. In some embodiments, the lithography system 10 is an EUVLsystem. For example, the lithography system may be designed to expose aphotoresist layer by EUV light or EUV radiation. The photoresist layeris a material sensitive to the EUV light. The lithography system 10employs a radiation source 12 to generate light, such as EUV lighthaving a wavelength ranging between about 1 nm and about 100 nm. In oneexample, the radiation source 12 generates an EUV light with awavelength centered at about 13.5 nm. Accordingly, the radiation source12 may be an EUV radiation source 12.

The lithography system 10 also employs an illuminator 14. In variousembodiments, the illuminator 14 includes various refractive opticcomponents, such as a single lens or a lens system having multiplelenses (zone plates) or alternatively reflective optics (for EUVlithography system), such as a single mirror or a mirror system havingmultiple mirrors, in order to direct light from the radiation source 12onto a mask stage 16, particularly to a mask 18 provided on the maskstage 16. In the present embodiment where the radiation source 12generates light in the EUV wavelength range, the illuminator 14 employsreflective optics. In some embodiments, the illuminator 14 includes adipole illumination component.

In some embodiments, the illuminator 14 is operable to configure themirrors to provide a proper illumination to the mask 18. In one example,the mirrors of the illuminator 14 are switchable to reflect EUV light todifferent illumination positions. In some embodiments, a stage prior tothe illuminator 14 may additionally include other switchable mirrorsthat are controllable to direct the EUV light to different illuminationpositions with the mirrors of the illuminator 14. In some embodiments,the illuminator 14 is configured to provide an on-axis illumination(ONI) to the mask 18. In an example, a disk illuminator 14 with partialcoherence of at most 0.3 is employed. In some other embodiments, theilluminator 14 is configured to provide an off-axis illumination (OAI)to the mask 18. In an example, the illuminator 14 is a dipoleilluminator. The dipole illuminator has a partial coherence of at most0.3 in some embodiments.

The mask stage 16 is configured to secure the mask 18. In someembodiments, the mask stage 16 includes an electrostatic chuck (e-chuck)to secure the mask 18. This is because gas molecules absorb EUV light,and the lithography system for the EUVL patterning is maintained in avacuum environment to avoid the EUV intensity loss. In the disclosure,the terms of mask, photomask, and reticle are used interchangeably torefer to the same item. The mask 18 is described in detail in FIG. 2.

The lithography system 10 may include a projection optics module (orprojection optics box (POB) 20 for imaging the pattern 52 of the mask 18on to a substrate 26 secured on a substrate stage 28 of the lithographysystem 10. The POB 20 has refractive optics (such as for UV lithographysystem) or alternatively reflective optics (such as for EUV lithographysystem) in various embodiments. The light directed from the mask 18,diffracted into various diffraction orders and carrying the image of thepattern 52 defined on the mask 18, is collected by the POB 20. The POB20 may include a magnification of less than one (thereby the size of the“image” on a target (such as the substrate 26 discussed below) issmaller than the size of the corresponding “object” on the mask 18). Theilluminator 14 and the POB 20 are collectively referred to as an opticalmodule of the lithography system 10.

The lithography system 10 may include a pupil phase modulator 22 tomodulate optical phase of the light directed from the mask 18 so thatthe light has a phase distribution on a projection pupil plane 24. Inthe optical module, there is a plane with field distributioncorresponding to Fourier Transform of the object (the mask 18 in thepresent case). This plane is referred to as projection pupil plane. Thepupil phase modulator 22 provides a mechanism to modulate the opticalphase of the light on the projection pupil plane 24. In someembodiments, the pupil phase modulator 22 includes a mechanism to tunethe reflective mirrors of the POB 20 for phase modulation. For example,the mirrors of the POB 20 are switchable and are controlled to reflectthe EUV light, thereby modulating the phase of the light through the POB20.

In some embodiments, the pupil phase modulator 22 utilizes a pupilfilter placed on the projection pupil plane. A pupil filter filters outspecific spatial frequency components of the EUV light from the mask 18.Particularly, the pupil filter is a phase pupil filter that functions tomodulate phase distribution of the light directed through the POB 20.However, utilizing a phase pupil filter is limited in some lithographysystem (such as an EUV lithography system) since all materials absorbEUV light.

As discussed above, the lithography system 10 also includes thesubstrate stage 28 to secure the substrate 26 to be patterned. Thesubstrate may be a semiconductor substrate. In some embodiments, thesubstrate 26 is a silicon substrate or other type of semiconductorsubstrate. The substrate 26 is coated with a resist layer sensitive tothe radiation, such as EUV light in some embodiments.

FIG. 2 is a cross-sectional view of the mask 18, in accordance with someembodiments. In some embodiments, the lithography system 10 is an EUVLsystem, and the mask 18 is a reflective mask. As shown in FIG. 2, themask 18 includes a substrate 40. The substrate 40 may include anysuitable material, such as a low thermal expansion material or fusedquartz. For example, the material of the substrate 40 includes TiO₂,doped SiO₂, or other suitable materials with low thermal expansioncoefficient. The mask 18 may further include a reflective multilayer(ML) 42 disposed on the substrate 40. The ML 42 includes alternatinglayers 44, 46. In some embodiments, the layer 44 includes Mo and thelayer 46 includes Si or Be. The layers 44, 46 may include any suitablematerial that is highly reflective with respect to EUV light. The mask18 may further include a capping layer 48, such as ruthenium (Ru),disposed on the ML 42 for protection. The mask 18 further includes anabsorption layer 50, such as a tantalum boron nitride (TaBN) layer,disposed over the ML 42. The absorption layer 50 includes a pattern 52to define a layer of an integrated circuit (IC). In some embodiments,the mask 18 includes a conductive backside coating 54. Alternatively,another reflective layer may be deposited over the ML 42 and ispatterned to define a layer of an integrated circuit, thereby forming anEUV phase shift mask.

After a period of time that the mask 18 is not being used in theprocess, a contamination layer 56 may be formed on the absorption layer50, as shown in FIG. 2. The contamination layer 56 may include carbonand/or oxide. For example, the mask 18 may be idle in the lithographysystem or in a mask storage, and the absorption layer 50 may bepartially oxidized, and the oxidized portion may be the contaminationlayer 56. The contamination layer 56 may cause a decrease in criticaldimension (CD) in EUV exposure when the mask 18 having the contaminationlayer 56 is used in process. The contamination layer 56 may be removedby increasing the energy of the light from the light source 12. However,in some other embodiments, a dummy substrate is used when removing thecontamination layer 56, because higher energy light may negativelyaffect the pattern formed on the substrate 26 if the thickness of thecontamination layer 56 is less than expected. In order to increasethroughput by eliminating the use of dummy substrate, a thicknessmeasuring device is provided in the lithography system 10.

Referring back to FIG. 1, the lithography system 10 further includes alight source 30 and a detector 32, as shown in FIG. 1. The light source30 and the detector 32 may be part of a thickness measuring device formeasuring a thickness of the contamination layer 56 (FIG. 2) formed onthe mask 18. The device may be any suitable thickness measuring device.For example, the device may utilize ellipsometry to measure thethickness of the contamination layer 56 on the mask 18. Ellipsometry isan optical technique for investigating properties of thin films, and canbe used to characterize thickness or depth. The change in polarizationis used as a signal after an incident light interacting with thecontamination layer 56. This is because that the polarization changedepends on the thickness of the contamination layer 56. In someembodiments, the light source 30 may be a visible light source orinvisible light source. For example, the light source 30 is a laser. Thelight source 30 may produce a light having one or more wavelengths, andthe light is directed to and reflected from the mask 18. The reflectedlight is detected by the detector 32, and the thickness of thecontamination layer 56 may be measured. Other components (not shown) ofthe thickness measuring device may include a polarizer and an analyzer.

The thickness measuring device described above utilizes ellipsometry tomeasure the thickness of the contamination layer 56. In someembodiments, other types of thickness measuring techniques may beutilized by the thickness measuring device, such as interferometry,reflectometry, picosecond ultrasonics, atomic force microscopy (AFM),scanning tunneling microscopy (STM), scanning electron microscopy (SEM),transmission electron microscopy (TEM), or other suitable techniques.

The thickness measuring device (i.e., the light source 30 and thedetector 32) is electrically connected to a controller 80, as shown inFIG. 1. The controller 80 controls the process of the thicknessmeasuring device. The controller 80 may be also electrically connectedto the light source 12 to control the energy of the light emitted by thelight source 12. The controller 80 may include a processor, a memory, atransmitter, and a receiver. In some embodiments, the controller 80 isconfigured to collect and analyze measured thickness data from thethickness measuring device (i.e., the light source 30 and the detector32), and to determine whether compensation energy should be applied tothe light emitted by the light source 12. The controller 80 may alsodetermine the amount of compensation energy to be applied to the lightemitted by the light source 12. For example, if the energy of the lightemitted by the light source 12 is set to be X mJ, and the controllerdetermines that a Y amount of compensation energy is to be applied tothe energy of the light emitted by the light source 12, the controller80 will increase the energy of the light to be emitted by the lightsource 12 to X+Y mJ. The amount of compensation energy may be determinedfrom data obtained for a particular type of the mask 18.

FIG. 3 is a chart showing a relationship between a thickness of thecontamination layer 56 and a decrease in CD in the pattern formed on thesubstrate 26, in accordance with some embodiments. As shown in FIG. 3,for a first type of the mask 18, such as mask type-A, 10 data pointsshowing an increase in the thickness of the contamination layer 56 leadsto a decrease (positive numbers on the y-axis) in CD in the patternformed on the substrate 26 using the first type of the mask 18. Thefitted line 302 shows a first relationship between the thickness of thecontamination layer 56 and the decrease in CD. For a second type of themask 18, such as mask type-B, 10 data points showing an increase in thethickness of the contamination layer 56 leads to a decrease in CD in thepattern formed on the substrate 26 using the second type of the mask 18.The fitted line 304 shows a second relationship between the thickness ofthe contamination layer 56 and the decrease in CD. As shown in FIG. 3,the first relationship is different from the second relationship. Thus,for different types of the mask 18, different relationships may bedetermined between the thickness of the contamination layer 56 and thedecrease in CD. The relationship may be linear, quadratic, cubic, orother suitable relationship. In some embodiments, a relationship of atype of the mask 18 between the thickness of the contamination layer 56and the decrease in CD is y=K/(1+be^(−ax)), where x is the thickness ofthe contamination layer 56, a, b, and K are constants from the fittedline (e.g., fitted line 302 or 304), and y is the decrease in CD.

After the relationship between the thickness of the contamination layer56 and the decrease in CD is identified for each type of the mask 18,the relationship between the decrease in CD and the amount ofcompensation energy is identified. For example, the relationship may befor every 1 nm decrease in CD, 1 mJ in compensation energy may beapplied. In some embodiments, the relationship between the decrease inCD and the amount of compensation energy is y=x, where y is the amountof compensation energy and x is the decrease in CD. As a result, arelationship of a type of the mask 18 between the thickness of thecontamination layer 56 and the amount of compensation energy isy=K/(1+be^(−ax)), where x is the thickness of the contamination layer56, a, b, and K are constants from the fitted line (e.g., fitted line302 or 304), and y is the amount of compensation energy.

Referring back to FIG. 1, with the relationship between the thickness ofthe contamination layer 56 and the amount of compensation energy to beapplied stored in the controller 80, the controller 80 can use thethickness information and the relationship to control the energy of thelight emitted by the light source 12. For example, the controller 80receives a signal from the thickness measuring device (i.e., thedetector 32) and determines the thickness of the contamination layer 56.From the relationship stored in the controller 80, for example,y=K/(1+be^(−ax)), where x is the thickness of the contamination layer56, a, b, and K are constants from the fitted line, the amount ofcompensation energy y can be calculated by the controller 80. Thecontroller 80 then controls the light source 12 so that the energy ofthe light emitted is increased by an amount equals to the amount ofcompensation energy y. The light emitted by the light source 12 with thecompensation energy applied removes the contamination layer 56 when thelight reaches the mask 18. In some embodiments, the thickness of thecontamination layer 56 is measured before a substrate 26 is processed.For example, before a substrate 26 is processed, the thickness measuringdevice is utilized to determine if a contamination layer 56 is formed onthe mask 18, and if so, what is the thickness of the contamination layer56. If the thickness of the contamination layer 56 is greater than athreshold value, compensation energy is applied to the light whenprocessing the substrate 26. The thickness measuring device may beutilized before every substrate 26 is being processed. The thicknessmeasuring device may perform in-situ inspection of the mask 18. In someembodiments, the thickness of the contamination layer 56 is measuredbefore a group (or lot) of substrates 26 is processed.

FIGS. 4A-4C are charts showing how compensate energy is applied toreduce variation in CD for a group of substrates, such as the substrates26, in accordance with some embodiments. As shown in FIG. 4A, a group ofsubstrates 26, or wafers, are processed using a single mask 18 withoutapplying compensation energy. The group of substrates 26 may include anynumber of substrates 26 greater than 1. In some embodiments, the groupof substrates 26 includes 151 substrates. The CDs of the substrates 26are plotted as the line 402. The line 402 (i.e., the CDs of the patternson the substrates 26) has high standard deviation (3σ is about 0.31).Specifically, the first wafer has a small CD due to the mask 18 beingidle (such as being disposed in a mask storage prior to be placed in thelithography chamber), the 19^(th) wafer has a small CD due to the mask18 being idle for about 8 hours, the 121^(st) wafer has a small CD dueto the mask 18 being idle for about 41 hours, and the 151^(st) wafer hasa small CD due to the mask 18 being idle for about 11 hours. Asdescribed above, the contamination layer 56 may be formed as a result ofthe mask 18 being idle. With the contamination layer 56, the CD of thepattern on the substrate 26 is decreased. Due to the 4 data pointsshowing small CDs, the overall consistency of the CDs of the group ofthe substrates 26 is decreased (i.e., higher standard variation). The CDbehavior represented by the line 402 for the particular type of the mask18 may be stored in the controller 80 (FIG. 1).

FIG. 4B is a chart showing how compensation energy is applied. With theknown relationship between the compensation energy and the thickness ofthe contamination layer 56 for the particular mask 18, for example,y=K/(1+be^(−ax)), where x is the thickness of the contamination layer56, a, b, and K are constants from the fitted line (e.g., fitted line302 or 304 shown in FIG. 3), and y is the amount of compensation energy,the compensation energy may be applied to the light emitted from thelight source 12 when processing one or more substrates 26 of the groupof the substrates 26. The right y-axis corresponds to the line 404,which is the amount of time the mask 18 is idle. The mask 18 may belocated in the lithography system 10 or a mask storage 600 (FIG. 6)while being idle. The left y-axis corresponds to the line 406, which isthe amount of compensation energy applied. For example, the mask 18 wasidle for about 11 hours prior to processing the first substrate, and thethickness measuring device (i.e., the light source 30 and the detector32) was used to measure a thickness of the contamination layer 56 formedon the mask 18 prior to processing the first substrate. With thethickness of the contamination layer 56 and the relationship between thethickness of the contamination layer 56 and the amount of compensationenergy, the controller 80 (FIG. 1) controls the light source 12 (FIG. 1)to emit a light with the compensation energy applied. The compensationenergy applied is represented by the line 406, which shows a gradualdecreasing until reaching 0. The gradual decreasing is based on the CDbehavior represented by the line 402 shown in FIG. 4A. The CDs of thesubstrates 26 gradually increases as the substrates 26 are processed,which means the contamination layer 56 is gradually removed by the lightemitted by the light source 12 without the compensation energy. Thus,the compensation energy gradually decreases to help remove thecontamination layer 56 without causing a surge in the CDs.

Based on the CD behavior stored in the controller 80, the controller 80knows when to add compensation energy to the light emitted by the lightsource 12 (FIG. 1). For example, the line 402 (FIG. 4A) shows thatbefore the 19^(th) substrate 26, the mask 18 is idle for about 8 hours,and the CD of the 19^(th) substrate 26 drops significantly. Thus, thecontroller 80 applies compensation energy to the light when processingthe 19^(th) substrate 26 in order to gradually remove the contaminationlayer 56 formed on the mask 18 during the 8 hours of the idle time.Similarly, the compensation energy gradually decreases until reaching 0in order to help remove the contamination layer 56 without causing asurge in the CDs.

Similarly, before processing the 121^(st) substrate 26, the mask 18 wasidle for about 41 hours, and compensation energy was applied whenprocessing the 121^(st) substrate 26 in order to gradually remove thecontamination layer 56 formed during the 41 hours of the idle time.Lastly, before processing the 151^(st) substrate 26, the mask 18 wasidle for about 11 hours, and compensation energy was applied whenprocessing the 151^(st) substrate 26 in order to gradually remove thecontamination layer 56 formed during the 11 hours of the idle time.

FIG. 4C is a chart showing the result CD variation of the group of thesubstrates after compensation energy is applied based on the methoddescribed in FIG. 4B. As shown in FIG. 4C, the CDs of the substrates 26are plotted as the line 408. The line 408 (i.e., the CDs of the patternson the substrates 26) has low standard deviation (3σ is about 0.20)compared to the standard deviation of line 402 shown in FIG. 4A.

The embodiments shown in FIGS. 4A-4C illustrates a method for measuringthe thickness of the contamination layer 56 formed on the mask 18 priorto processing a group of substrates 26. If the thickness of thecontamination layer 56 is over a threshold value, compensation energy isapplied when processing a first subgroup of substrates 26, and thecompensation energy gradually decreases until reaching 0. Subsequently,with the CD variation data without compensation energy stored in thecontroller 80, the controller 80 determines when to apply compensationbased on the stored CD variation data. In other words, the thicknessmeasuring device is utilized once for every group of substrates 26.

FIGS. 5A and 5B are schematic views of the lithography system 10, inaccordance with alternative embodiments. As shown in FIG. 5A, in someembodiments, the lithography system 10 includes a load port 101, atransferring module 102, a transportation stage 104, a processingapparatus 105, and a controller 107. Elements of the lithography system10 can be added to or omitted, and the disclosure should not be limitedby the embodiments. In some embodiments, the lithography system 10 is anEUVL system and the processing apparatus 105 is an EUVL apparatus.

The transferring module 102 is configured to transfer a mask 200 betweenthe load port 101 and the transportation stage 104. The mask 200 may bethe mask 18 shown in FIG. 1. In some embodiments, the transferringmodule 102 is positioned between the load port 101 and thetransportation stage 104. The transferring module 102 may include acontrol circuit 1021 and a robotic arm 1023. The control circuit 1021 isconfigured to generate an electrical signal to the robotic arm 1023, soas to control the robotic arm 1023 to transfer the mask 200. In someembodiments, the robotic arm 1023 may include a six-axis robotmanipulator and is configured to hold the mask 200.

In some embodiments, the transportation stage 104 is used for conveyingthe mask 200 into the processing apparatus 105. As shown in FIG. 5A, theprocessing apparatus 105 can include a lithography chamber 105A and adevice chamber 105B. One or more openings 105C may be formed in thechamber wall separating the lithography chamber 105A and the devicechamber 105B. The lithography chamber 105A can include a mask stage 300,a substrate stage 1051 for supporting a substrate 400, and a projectionoptics module 260 (POB). The substrate 400 may be the substrate 26, themask stage 300 may be the mask stage 16, the substrate stage 1051 may bethe substrate stage 28, and the POB 260 may be the POB 20 described inFIG. 1.

As shown in FIG. 5A, in some embodiments, a light source 250 and athickness measuring device 265 are disposed in the device chamber 105B.The light source 250 may be the light source 12 described in FIG. 1 andthe thickness measuring device 265 may be the thickness measuring device(i.e., the light source 30 and the detector 32) described in FIG. 1. Insome embodiments, the thickness measuring device 265 includes a lightsource, a detector, and other components, such as optical components.The thickness measuring device 265 is configured to perform in-situmeasurement of the thickness of the contamination layer 56 formed on themask 200. The thickness measuring device 265 may be utilized in the samemanner as the thickness measuring device described in FIG. 1. The maskstage 300 is coupled to a shaft 270, and the shaft 270 is configured totilt the mask stage 300 with respect to a plane substantially parallelto the top surface of the substrate stage 1051. The position of the maskstage 300 shown in FIG. 5A is set for a lithography process, where alight emitted from the light source 250 reaches the mask 200 andreflects to the substrate 400. FIG. 5B illustrates an embodiment wherethe mask stage 300 is tilted towards the thickness measuring device 265for the thickness measuring device 265 to determine the thickness of thecontamination layer 56 on the mask 200. The tilting of the mask stage300 by the shaft 270 enables the light source and the detector of thethickness measuring device 265 to be located at the same position. Themask stage 300 may be tilted for the thickness measuring device 265 toinspect the mask 200 before processing each substrate 400 or beforeprocessing a group of substrates 400.

As shown in FIGS. 5A and 5B, the lithography system 10 may furtherinclude a controller 107 for control the operation of the lithographysystem 10. The controller 107 may be the controller 80 described inFIG. 1. In some embodiments, the controller 107 receives the thicknessof the contamination layer 56 on the mask 200 from the thicknessmeasuring device 265 and controls the light source 250 so compensationenergy may be applied to the light emitted from the light source 250 inorder to remove the contamination layer 56. As described above, thethickness measuring device 265 may be utilized to inspect the mask 200before every substrate 400 being processed or before every group ofsubstrates 400 being processed.

FIG. 6 is a schematic top view of a mask storage 600, in accordance withsome embodiments. The mask storage 600 may be part of the lithographysystem 10. As shown in FIG. 6, the mask storage 600 includes one or morearrays of storage units 602. Each storage unit 602 may be configured tostore one or more masks 18 (or masks 200). A gantry 604 is disposed overthe space between the arrays of storage units 602. The gantry 604includes a rail 606 supported by supports 608. A robotic arm 610 isdisposed on the rail 606 of the gantry 354. The robotic arm 610 isconfigured to transfer the mask 18 in and out of the mask storage 600.The mask storage 600 further includes a stage 612 and a thicknessmeasuring device 614. Prior to be placed in the processing apparatus 105(FIGS. 5A and 5B), the mask 18 (or the mask 200) may be placed on thestage 612 by the robotic arm 610, and the thickness measuring device 614measures the thickness of the contamination layer 56 formed on the mask18. The thickness measuring device 614 may be the thickness measuringdevice 265 described in FIGS. 5A and 5B. The thickness measuring device614 may be electrically connected to a controller, such as thecontroller 80 or the controller 107, and the thickness information issent to the controller. With the thickness of the contamination layer 56determined, the controller can apply compensation energy to the light toremove the contamination layer 56 from the mask 18.

The thickness measuring device, such as the light source 30 and thedetector 32 shown in FIG. 1, the thickness measuring device 265 shown inFIGS. 5A and 5B, or the thickness measuring device 614 shown in FIG. 6,are utilized to measure the thickness of the contamination layer 56formed on the mask 18 (or the mask 200). The thickness of thecontamination layer 56 may be measured before processing a substrate 26(or a substrate 400) or measured before processing a group of substrates26 (or substrates 400). Compensation energy may be applied to the lightbefore and/or during the processing of one or more substrates 26 (orsubstrates 400) in order to remove the contamination layer 56 from themask 18 (or the mask 200). The thickness of the contamination layer 56may be measured in the processing apparatus as shown in FIGS. 1, 5A, and5B, or in the mask storage 600 as shown in FIG. 6. The thickness may bemeasured in any location along the path of the mask 18 (or the mask 200)from the mask storage 600 to the processing apparatus. In someembodiments, a thickness measuring device may be located over thetransportation stage 104 (FIGS. 5A and 5B). In some embodiments,multiple thickness measuring devices may be located along the path ofthe mask 18. For example, the thickness measuring device 614 and thethickness measuring device 265 may be both utilized.

The present disclosure in various embodiments provides a lithographysystem having a thickness measuring device for measuring a thickness ofa contamination layer formed on a mask. The thickness measuring devicemay be located in the processing apparatus or in the mask storage. Thethickness information is transformed to a signal of compensation energy,which may be applied to the light before each exposure (substrate tosubstrate) or before a group of exposures (lot to lot). Some embodimentsmay achieve advantages. For example, CD variation may be improved.Furthermore, no dummy substrate is used, which improves throughput.

An embodiment is a method. The method includes obtaining a relationshipbetween thicknesses of a contamination layer formed on a mask andamounts of compensation energy to remove the contamination layer,obtaining a first thickness of a first contamination layer formed on themask from a thickness measuring device, and applying first compensationenergy calculated from the relationship to a light directed to the mask.

Another embodiment is a lithography process. The process includesperforming the lithography process in a lithography system. Thelithography system includes a first light source, a mask stage, asubstrate stage, and a thickness measuring device configured to measurea thickness of a contamination layer formed on a mask to be disposed onthe mask stage.

A further embodiment is a lithography system. The system includes alight source, a mask stage, a substrate stage, and a mask storage. Themask storage includes a plurality of storage units and a first thicknessmeasuring device configured to measure a thickness of a contaminationlayer formed on a mask to be disposed in the mask storage.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. A method, comprising: obtaining a relationship between thicknesses ofa contamination layer formed on a mask and amounts of compensationenergy to remove the contamination layer; obtaining a first thickness ofa first contamination layer formed on the mask from a thicknessmeasuring device; and removing the first contamination layer by applyingfirst amounts of compensation energy calculated from the relationship toa first light directed to the mask, wherein each amount of compensationenergy of the first amounts of compensation energy is less than aprevious amount of compensation energy of the first amounts ofcompensation energy.
 2. The method of claim 1, further comprisingmeasuring the first thickness of the first contamination layer by thethickness measuring device.
 3. The method of claim 2, wherein themeasuring the first thickness of the first contamination layer isperformed in a processing apparatus.
 4. The method of claim 2, whereinthe measuring the first thickness of the first contamination layer isperformed in a mask storage.
 5. The method of claim 1, furthercomprising forming a second contamination layer on the mask afterapplying the first amounts of compensation energy, wherein the secondcontamination layer is formed while the mask is idle for a period oftime.
 6. The method of claim 5, wherein the period of time is greaterthan about eight hours.
 7. The method of claim 5, wherein the mask isdisposed in a lithography system while being idle.
 8. The method ofclaim 7, further comprising applying second amounts of compensationenergy calculated from the relationship to a second light directed tothe mask.
 9. The method of claim 8, wherein the second contaminationlayer is removed by the second amounts of compensation energy.
 10. Alithography process, comprising: performing the lithography process in alithography system, the lithography system comprising: a first lightsource; a mask stage; a substrate stage; a thickness measuring deviceconfigured to measure a thickness of a contamination layer formed on amask to be disposed on the mask stage; and a controller configured tocontrol the first light source to emit a light with gradually decreasingamounts of compensation energy to remove the contamination layer whileperforming the lithography process.
 11. The lithography process of claim10, wherein the thickness measuring device comprises a second lightsource and a detector.
 12. The lithography process of claim 11, furthercomprising: an illuminator disposed between the first light source andthe mask stage; and a projection optics box disposed between the maskstage and the substrate stage.
 13. The lithography process of claim 10,wherein the mask stage and the substrate stage are disposed in alithography chamber, and the first light source and the thicknessmeasuring device are disposed in a device chamber adjacent thelithography chamber.
 14. The lithography process of claim 13, furthercomprising a transferring module and a transportation stage, wherein thetransportation stage is disposed between the transferring module and thelithography chamber.
 15. The lithography process of claim 13, furthercomprising a shaft coupled to the mask stage, wherein the shaft isconfigured to tilt the mask stage with respect to a plane substantiallyparallel to a top surface of the substrate stage.
 16. The lithographyprocess of claim 10, wherein the lithography process is an extremeultraviolet lithography process.
 17. A lithography system, comprising: alight source configured to expose a photoresist layer; a mask stage; asubstrate stage; a mask storage, comprising: a plurality of storageunits; and a first thickness measuring device configured to measure athickness of a contamination layer formed on a mask to be disposed inthe mask storage; and a controller configured to control the lightsource to emit a light with gradually decreasing amounts of compensationenergy to remove the contamination layer.
 18. The lithography system ofclaim 17, further comprising: an illuminator disposed between the lightsource and the mask stage; and a projection optics box disposed betweenthe mask stage and the substrate stage.
 19. The lithography system ofclaim 18, wherein the mask stage and the substrate stage are disposed ina lithography chamber, and the light source is disposed in a devicechamber adjacent the lithography chamber.
 20. The lithography system ofclaim 19, further comprising a second thickness measuring devicedisposed in the device chamber, wherein the second thickness measuringdevice is electrically connected to the controller.